Stretchable supercapacitors with vertically-aligned embedded carbon nanotubes

ABSTRACT

Flexible and stretchable supercapacitors are made using carbon nanostructures produced by providing a first composite structure which includes a temporary substrate and an array of carbon nanotubes arranged in a stack on a surface of the temporary substrate such that the stack of carbon nanotubes is oriented generally perpendicular to the surface of the temporary substrate, which may include silicon dioxide. The stack of carbon nanotubes is transferred from the temporary substrate to another substrate, which includes a curable polymer, thereby forming another composite structure comprising the stack of carbon nanotubes and the cured polymer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/500,041 filed May 2, 2017, the entire disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of supercapacitors andmethods of making supercapacitors, and, more specifically, to flexibleand stretchable supercapacitors that include carbon nanostructures.

BACKGROUND OF THE INVENTION

The increasing depletion of fossil fuels and the environmental problemscaused by those fossil fuels have motivated researchers to develop newtypes of clean energy, such as solar, wind, and water energy. As thesenew energy sources are limited by time or location (water, wind, andsolar power), energy storage devices are required to ensure continuedpower supply. Supercapac tors, as one type of energy storage device,have received intensive attention owing to their high power density,fast charge/discharge, and long charge/discharge cycles.

Flexible electronics have a wide range of applications in wearable andmultifunctional electronics, including flexible displays, electronicskins, and implantable medical devices. Flexible supercapacitors, withgood mechanical compliance, can meet the requirements for light-weight,portable and flexible devices.

Carbon nanotubes are a promising electrode material for flexiblesupercapacitors owing to their excellent properties. However, thefabrication of flexible supercapacitors in large quantities can be acomplicated process. For example, to apply electrode materials ontoflexible substrates, researchers have used direct-coating methods whichrely heavily relies on the physical adhesion of the electrode materialson the substrate, or stacking the electrode and electrolyte. It has beenfound, however, that the electrode/substrate interface resulting fromsuch fabrication methods can delaminate under large strain, therebylimiting the flexibility of the supercapacitors produced thereby andconsequently deteriorating their performance.

SUMMARY OF THE INVENTION

In one embodiment, a method according to the present invention enablesthe facile fabrication of flexible supercapacitors usingpolydimethylsiloxane (PDMS) to infiltrate between an array of carbonnanotubes, thereby achieving strong adhesion between the PDMS and thevertically aligned carbon nanotubes (VACNTs) due to the viscoelasticproperty of PDMS which promotes the adhesion between the VACNTs andPDMS. In accordance with this embodiment, the present invention enablesfacile fabrication of flexible supercapacitors at a high rate ofthroughput,

In accordance with another embodiment of the present invention, aflexible and stretchable supercapacitor includes VACNTs in a curablepolymer (e.g., PDMS), with the VACNTs/PDMS composite structurefunctioning as an electrode for flexible supercapacitors. Afterassembling the electrode with liquid or solid electrolyte, the flexiblesupercapacitors, acting as energy storage devices, can be integratedinto flexible electronics. In such applications of the presentinvention, the VACNTs/PDMS composite structures produced in accordancewith the present invention maintain their structural integrity undertensile strains over 1000 charge/discharge cycles without majordegradation of their functionality.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the present invention, reference is madeto the following detailed description of various exemplary embodimentsconsidered in conjunction with the accompanying Figures, in which likestructures are referred to by the like reference numerals throughout theseveral Figures, and in which:

FIGS. 1(a)-(c) represent a sequence of schematic illustrations depictingone example of a process that can be employed to construct a VACNT/PDMSstack for use in flexible supercapacitors fabricated according to anembodiment of the present invention;

FIGS. 2(a)-(d) represent a set of SEM images of CVD-grown VACNTs on asilicon dioxide (SiO₂) substrate (see FIG. 2(a) and FIG. 2(b)), andVACNTs successfully transferred on PDMS (see FIG. 2(c) and FIG. 2(d)),in which the structure shown in the SEM images was obtained bydelaminating the VACNT/PDMS stack from the SiO₂ substrate during afabrication process according to an embodiment of the present invention;

FIGS. 3(a)-(d) represent a group of charts illustrating anelectrochemical characterization of a VACNTs/PDMS stack under tensilestrain and bending, and more particularly, showing: in FIG. 3(a), achart of cyclic voltammetry data at a scan rate of 1 V/s; in FIG. 3(b),a chart of capacitances at scan rates ranging from 50 mV/s to 1 V/s; inFIG. 3(c), a chart of capacitances under strains of up to 200% at thescan rates of 100 mV/s and 1 V/s; and in FIG. 3(d), a chart ofcapacitances under bending, the angles of bending ranging from 0 to 180degrees.

DETAILED DESCRIPTION OF THE INVENTION

With initial and general reference to FIGS. 1-3 and the accompanyingtext which follows, it is noted that terms indicating position,orientation or direction of motion are used throughout the discussion ofFIGS. 1-3 for the purpose of facilitating discussion only, and not tolimit the disclosed embodiments to the particular terms describedherein. Nor do they limit the physical orientation in actual use to anyparticular coordinate system (e.g., horizontal, vertical and front, backand side).

It should also be noted that the following disclosure is presented toprovide an illustration of the general principles of the presentinvention and is not meant to limit, in any way, the inventive conceptsdisclosed and claimed herein. All terms defined herein should beafforded their broadest possible interpretation, including any impliedmeanings as dictated by a reading of the specification as well as anywords that a person having skill in the art and/or a dictionary,treatise, or similar authority would assign thereto.

Further, it should be noted that, as recited herein, the singular forms“a”, “an”, and “the” include the plural referents unless otherwisestated. Additionally, the terms “comprises”, “comprising”, “includes”,“including”, “has” and the like, when used herein specify that certainfeatures are present in that embodiment, however, such terms should notbe interpreted to preclude the presence or addition of additional steps,operations, features, components, and/or groups thereof.

With the foregoing prefatory comments in mind, embodiments of thepresent invention include a facile fabrication method utilizing VACNTcarpets. Such a method enables high-throughput fabrication ofsupercapacitors that are flexible and stretchable. The inventive methodprovides a strong adhesion between VACNT carpets and PDMS, whichfacilitates a stable charge/discharge cycle under various tensile strainconditions. Such performance characteristics enhance the practicality ofincluding the VACNTs/PDMS structures of the present invention inflexible supercapacitors. The VACNTs/PDMS structures possess a very highsurface area, which contributes to the unexpectedly high capacitance ofthe flexible supercapacitors produced in accordance with the presentinvention.

Referring initially to FIGS. 1(a)-(c), the first step of an exemplaryembodiment of a fabrication method according to the present inventioninvolves growing a dense carpet-like structure of VACNTs 10 on awafer-like SiO₂/Si substrate 12 using atmospheric pressure chemicalvapor deposition (APCVD). More particularly, the SiO₂/Si substrate 12illustrated schematically in FIGS. 1(a)-(c) is a SiO₂/Si wafer with 5 nmthick Al and 3 nm thick Fe as catalyst deposited on its surface byphysical vapor deposition (PVD). After placing the SiO₂/Si substrate 12in an APCVD chamber (not shown), its furnace temperature is increased to750° C. with 500 sccm Ar flow. VACNTs are grown in the APCVD chamber at750° C. for 15 minutes ith 60 sccm H₂ and 100 sccm C₂H₄. The APCVDchamber is then cooled down to room temperature while keeping the Arflowing. The resulting dense carpet-like structure of VACNTs 10 (seeFIGS. 2(a)-(d)) formed by this exemplary method are aligned verticallyrelative to the surface of the SiO₂/Si substrate 12, which substrate 12is considered for the purpose of the present example to be horizontal,thereby forming a composite structure 14 consisting essentially of thecarpet-like structure of VACNTs 10 and the SiO₂/Si substrate 12. Inembodiments of the present invention, other methods of forming VACNTsmay be used in place of the exemplary method discussed above.

The next step of the exemplary method is to transfer the carpet-likestructure of VACNTs 10 onto PDMS or another polymer before the polymerfully cures. To form a suitable PDMS structure 16, a PDMS base and asuitable curing agent (e.g., Sylgard 184 Silicone Elastomer, DowCorning) are mixed in a ratio of 10:1 (PDMS base:curing agent), anddegassed under reduced pressure in a vacuum pump to remove bubbles fromthe liquid mixture. The liquid mixture is then heated on a hot plate at65° C. for about 30 minutes. The previously formed composite structure14 (i.e., the carpet-like structure of VACNTs 10 and the SiO₂/Sisubstrate 12) is placed face-to-face onto the PDMS structure 16 beforethe PDMS is fully cured. Once the PDMS is fully cured, the result is aVACNTs/PDMS composite structure 18 that can be peeled off (i.e.,delaminated) from the SiO₂/Si substrate 12.

In embodiments of the present invention, the VACNTs/PDMS compositestructure 18 functions as an electrode for flexible supercapacitors,with the electrolyte S for such flexible supercapacitors being either anionic-liquid or a solid. In an exemplary embodiment of the presentinvention, a solid electrolyte can be fabricated by mixing polyvinylalcohol powder and potassium hydroxide (KOH) in deionized water, whilealso evaporating the excess water to obtain a gel electrolyte. To createall-solid-state flexible supercapacitors according to embodiments of thepresent invention, the gel electrolyte is sandwiched between a pair ofthe VACNTs/PDMS composite structures 18.

Referring to FIGS. 3(a)-(d), the electrochemical properties of theVACNTs/PDMS composite structures 18 made using methods according to thepresent invention were measured in 30% KOH using cyclic voltammetry (CV)in a three electrode configuration. Platinum (Pt) foil was used as acounter electrode and Ag/AgCl (saturated KCl) as the referenceelectrode. CV measurements were performed within the potential range of0.0V-0.5V at scan rates of 50-1000 mV/s. The capacitances of theelectrodes were calculated as a capacitance per area (F/cm²). Theaverage capacitance was normalized per area of the samples and wasestimated according to the following equation.

$C = \frac{\int_{E_{1}}^{E_{2}}{I\mspace{14mu} {dV}}}{V \times \Delta \; V \times A}$

where I is the current, A is the area of he supercapacitor, ΔV is thescanning rate, E₁ and E₂ are the voltage and V=E₂−E₁.

To evaluate the flexibility and durability of the VACNTs/PDMS compositestructures 18, both tensile strain measurements and bending strainmeasurements were performed. Such measurements were made as theVACNTs/PDMS composite structures 18 were stretched from 0% to 20% andbent from 0 to 180 degrees.

The VACNTs/PDMS composite structures 18 exhibited good electrochemicalstability and capacitive behaviors at scanning rates from 50 mV/s to 1V/s. The measured capacitance (see FIG. 3(a) and FIG. 3(b)), which hasan area of 0.54 cm², was approximately 170 μF/cm² at a high scan rate of1 V/s. In addition, the strong adhesion between the VACNTs and the PDMSenabled the VACNTs/PDMS structures 18 to sustain various bending andtensile strains (see FIG. 3(c) and FIG. 3(d)). The VACNTs/PDMS compositestructures 18 could be bent up to 180 degrees, and the capacitance undersuch strains was consistent under bending angles in the range of 0 to180 degrees. The maximum tensile strain was 200% (see FIG. 3(c)).Further tests have demonstrated that the capacitance of the VACNTs/PDMScomposite structures 18 can remain consistent under tensile strains ofat least 300%.

Flexible supercapacitors that include the VACNTs/PDMS compositestructures 18 made using methods according to embodiments of the presentinvention are expected to have applications in, for example, the fieldsof wearable electronics, flexible photovoltaics (e.g., rolled-updisplays), self-powered wearable optoelectronics, and electronic skins.

It will be understood that the embodiments of the present inventiondescribed herein are merely exemplary and that a person skilled in theart may make many variations and modifications without departing fromthe spirit and scope of the invention. All such variations andmodifications are intended to be included within the scope of theinvention, as defined in the following claims.

1-20. (canceled)
 21. A method of fabricating a flexible supercapacitor,comprising the steps of: providing a first composite structure whichincludes a first substrate and an array of carbon nanotubes arranged ina stack on a surface of said first substrate such that said stack ofcarbon nanotubes is oriented generally perpendicular to said surface ofsaid first substrate and such that each of said carbon nanotubes has afirst end embedded in said first substrate and a second end distal tosaid first substrate; providing a second substrate which includes acurable polymer; partially curing said polymer; placing said stack ofcarbon nanotubes into contact with said second substrate such that saidsecond end of each of said carbon nanotubes is partially embedded insaid second substrate while said polymer is partially cured; fullycuring said polymer while said second end of each of said carbonnanotubes is partially embedded in said second substrate, whereby asecond composite structure is formed which includes said firstsubstrate, said second substrate and said stack of carbon nanotubesbetween said first and second substrates; delaminating said firstsubstrate from said stack of carbon nanotubes to thereby form a thirdcomposite structure which includes said stack of carbon nanotubes andsaid second substrate; and incorporating said third composite structureinto an electrode adapted for use in a flexible supercapacitor.
 22. Themethod of claim 21, wherein said first substrate includes silicon. 23.The method of claim 21, wherein said first substrate includes silicondioxide.
 24. The method of claim 21, wherein said first substrate has awafer-like structure made from silicon or silicon dioxide.
 25. Themethod of claim 24, wherein said wafer-like structure includes acatalyst which is applied thereto by physical vapor deposition.
 26. Themethod of claim 25, wherein said catalyst is selected from the groupconsisting of aluminum and iron.
 27. The method of claim 21, whereinsaid stack of carbon nanotubes is grown on said first substrate in anatmospheric pressure chemical vapor deposition chamber.
 28. The methodof claim 27, wherein said stack of carbon nanotubes is grown at atemperature of 750° C. for 15 minutes.
 29. The method of claim 28,wherein said stack of carbon nanotubes is grown into a dense carpet-likestructure.
 30. The method of claim 21, wherein said second substrateincludes a curing agent for said curable polymer.
 31. The method ofclaim 30, wherein said polymer is polydimethylsiloxane and said curingagent is a silicone elastomer.
 32. The method of claim 21, wherein saidpolymer, once fully cured, adheres to said second ends of said carbonnanotubes.
 33. The method of claim 21, wherein said electrode iscombined with an electrolyte to form said flexible supercapacitor. 34.The method of claim 33, wherein said electrolyte is an ionic liquid. 35.The method of claim 33, wherein said electrolyte is a solid.
 36. Themethod of claim 33, wherein said electrolyte is a gel.
 37. The method ofclaim 33, wherein said electrolyte comprises polyvinyl alcohol powder,potassium hydroxide and deionized water.
 38. The method of claim 21,wherein said carbon nanotube structure maintains structural integrityunder tensile strains up to 300%.
 39. The method of claim 21, whereinsaid carbon nanotube structure retains its functionality over 1000charge and discharge cycles.
 40. The method of claim 21, wherein saidpolymer has viscoelastic properties.